The mature central nervous system exhibits the capacity to alter cellular interactions as a function of the activity of specific neuronal circuits. This capacity is believed to underlie learning and memory storage, age-related memory loss, tolerance to and dependence on drugs of abuse, recovery from brain injury, epilepsy as well as aspects of postnatal development of the brain (Schatz, C., Neuron, 5:745, 1990). Currently, the role of activity-dependent synaptic plasticity is best understood in the context of learning and memory. Cellular mechanisms underlying activity-dependent plasticity are known to be initiated by rapid, transmitter-induced changes in membrane conductance properties and activation of intracellular signaling pathways (Bliss and Collingridge, Nature, 361:31, 1993). Several lines of evidence also indicate a role for rapid synthesis of mRNA and protein in long-term neuroplasticity. For example, classical studies of learning and memory demonstrate a requirement for protein synthesis in long-term, but not short-term memory (Flexner, et al., Science, 141:57, 1963; Agranoff, B., Basic Neurochemistry, 3rd Edition, 1981; Davis and Squire, Physiol. Bull., 96:518, 1984), and long-term enhancement of synaptic connectivity, studied in cultured invertebrate neurons (Montarolo, et al., Science, 234:1249, 1986; Bailey, et al., Neuron, 9:749, 1992) or in the rodent hippocampus (Frey, et al., Science, 260:1661, 1993; Nguyen, et al., Science, 265:1104, 1994), is blocked by inhibitors of either RNA or protein synthesis. Importantly, inhibitors of macromolecular synthesis are most effective when administered during a brief time window surrounding the conditioning stimulus indicating a special requirement for molecules that are rapidly induced (Goelet, et al., Nature, 322:419, 1986).
Immediate early genes (IEGs) are rapidly induced in neurons by neurotransmitter stimulation and synaptic activity and are hypothesized to be part of the macromolecular response required for long-term plasticity (Goelet, et al., supra; Sheng and Greenberg, Neuron, 4:477, 1990; Silva and Giese, Neurobiology, 4:413, 1994). To identify cellular mechanisms that may contribute to long-term plasticity in the vertebrate brain, differential cloning techniques have been used to identify genes that are rapidly induced by depolarizing stimuli (Nedivi, et al., Nature, 363:713, 1993; Qian, et al., Nature, 361:453, 1993; Yamagata, et al., Neuron, 11:371, 1993; Yamagata, et al., Learning and Memory 1:140, 1994; Yamagata, et al., Journal of Biological Chemistry, 269:16333, 1994; Andreasson and Worley, Neuroscience, 69:781, 1995; Lyford, et al., Neuron, 14:433, 1995). In contrast to the earlier focus on transcription factors, many of the newly characterized IEGs represent molecules that can directly modify the function of cells and include growth factors (Nedivi, et al., supra; Andreasson and Worley, supra ), secreted enzymes that can modify the extracellular matrix, such as tissue plasminogen activator (Qian, et al., supra), enzymes involved in intracellular signaling, such as prostaglandin synthase (Yamagata, et al., supra), and a novel homolog of H-Ras, termed Rheb (Yamagata, et al., supra), as well as a novel cytoskeleton-associated protein, termed Arc (Lyford, et al., supra). The remarkable functional diversity of this set of rapid response genes is representative of the repertoire of cellular mechanisms that are likely to contribute to activity-dependent neuronal plasticity.
Pharmaceutical agents often act by modulating signaling between cells or within cells. For example, Prozac alters the reuptake of the neurotransmitter serotonin and enhances aspects of its signaling function in brain. Nonsteroidal antiinflammatory drugs (NSAIDs) act by inhibiting the activity of cyclooxygenase enzyme, which is involved in the signaling pathways of inflammation. Viagra modifies the intracellular guanylate cyclase response to autonomic neurotransmitters in erectile tissues. These, and other precedent setting pharmaceuticals, validate the notion that specific signaling pathways may be targeted for therapeutic development.
Cellular mechanisms that modify important intracellular signals can involve changes in intracellular calcium. This type of mechanism is used in brain neurons to adapt to changes in intercellular signaling, and is demonstrated to exert powerful effects on cellular responses induced by glutamate. Similar, though distinct, cellular mechanism may be used to modulate intracellular calcium signals in other tissues including heart, lung, liver and skeletal muscle. Compounds that can modify this mechanism can modulate natural transmitter signals and may exert therapeutic effects.
Classical studies demonstrated that activation of receptors on the cell surface evoke changes in the level of specific, diffusable molecules inside the cell. The regulated production of these molecules serves to signal events happening at the membrane surface to intracellular receptors and are therefore termed second messenger signaling pathways. Major second messenger pathways include the phosphoinositide pathway, which regulates intracellular calcium; the adenylate cyclase pathway, which regulates levels of cyclic AMP; the guanylate cyclase pathway, which regulates levels of cGMP; and the nitric oxide pathway which regulates NO.
The regulated release of intracellular calcium is essential to the function of all tissues. Each tissue possesses a distinct physiology that is dependent on receptor/transmitter-regulated release of intracellular calcium. For example, synaptic function is modulated in brain neurons by glutamate receptor regulated release of intracellular calcium. Contractility of cardiac and smooth muscle is also regulated by intracellular calcium. Recent reviews of the role of calcium signaling in cellular responses include: Berridge, Nature 386:759 (1997); Berridge, J. Physiol. (London) 499:291 (1997); Bootman et al., Cell 91:367 (1997).
Recent studies demonstrate that molecules that function together in signaling networks are frequently clustered together in macromolecular complexes. For example, components of the MAP kinase pathway form a complex of cytosolic kinases with their specific substrates (Davis, Mol. Reprod. Dev. 42:459 (1995)). Similarly, proteins such as AKAP function as scaffolds for specific kinases and their substrates (Lester and Scott, Recent Prog. Horm. Res. 52:409 (1997)). Recently, a multi-PDZ containing protein was identified in Drosophila (termed InaD) that couples the membrane-associated, light-activated ion channel with its effector enzymes (Tsunoda et al., Nature 388:243 (1997)). The biochemical consequence of this clustering is that the local concentrations of molecules that convey the signals between proteins are as high as possible. Consequently, signaling takes place efficiently. The clustering activity of these proteins is essential to normal function of the signaling cascade (Lester and Scott, supra 1997; Tsunoda et al., supra 1997). Accordingly, Accordingly, agents that alter these signaling complexes will modify the response due to transmitter or other form of cellular stimulation in a way that mimics more classical receptor agonists or antagonists. For example, a metabotropic glutamate receptor signaling may be blocked either at the receptor by conventional receptor antagonists or by uncoupling the metabotropic receptor from its intracellular IP3 receptor by agents that block the cross-linking activity of Homer family proteins.
The identification of molecules regulating the aggregation of neurotransmitter receptors at synapses is central to understanding the mechanisms of neural development, synaptic plasticity and learning. The most well characterized model for the synaptic aggregation of ionotropic receptors is the neuromuscular junction. Early work showed that contact between the axon of a motor neuron and the surface of a myotube rapidly triggers the accumulation of preexisting surface acetylcholine receptors (Anderson and Cohen, J Physiol 268:757-773, 1977; Frank and Fischbach, J Cell Biol 83:143-158, 1979). Subsequent work has shown that agrin, a complex glycoprotein secreted by the presynaptic terminal, activates a postsynaptic signal transduction cascade (reviewed by Colledge and Froehner, Curr Opin Neurobiol 8:357-63, 1998), that leads to receptor clustering by the membrane associated protein rapsyn.